Optimize waveform stop routines (#4920)

Thanks to ideas from @shimarin for offering ideas to speed up the stopWaveform
calls which may help things like SoftwareSerial run better.

Optimize the stopWaveform routine to abort fast and early whenever possible.

Remove the stopWaveform call from digitalRead().  If you're running a waveform
on a pin and try to read it, that is a logic error and you'll end up reading the
waveform and not the outside world's view of the pin.

Author: earlephilhower

cores/esp8266/core_esp8266_waveform.c

@@ -63,8 +63,8 @@
 typedef struct {
   uint32_t nextServiceCycle;        // ESP cycle timer when a transition required
   uint32_t timeLeftCycles;          // For time-limited waveform, how many ESP cycles left
-  uint16_t gpioMask;                // Mask instead of value to speed IRQ loop
-  uint16_t gpio16Mask;              // Mask instead of value to speed IRQ loop
+  const uint16_t gpioMask;          // Mask instead of value to speed IRQ loop
+  const uint16_t gpio16Mask;        // Mask instead of value to speed IRQ loop
   unsigned state              : 1;  // Current state of this pin
   unsigned nextTimeHighCycles : 31; // Copy over low->high to keep smooth waveform
   unsigned enabled            : 1;  // Is this GPIO generating a waveform?
@@ -204,13 +204,21 @@ int startWaveform(uint8_t pin, uint32_t timeHighUS, uint32_t timeLowUS, uint32_t
 
 // Stops a waveform on a pin
 int stopWaveform(uint8_t pin) {
+  // Can't possibly need to stop anything if there is no timer active
+  if (!timerRunning) {
+    return false;
+  }
+
   for (size_t i = 0; i < countof(waveform); i++) {
-    if ((((pin == 16) && waveform[i].gpio16Mask) || ((pin != 16) && (waveform[i].gpioMask == 1<<pin))) && waveform[i].enabled) {
+    if (!waveform[i].enabled) {
+      continue; // Skip fast to next one, can't need to stop this one since it's not running
+    }
+    if (((pin == 16) && waveform[i].gpio16Mask) || ((pin != 16) && (waveform[i].gpioMask == 1<<pin))) {
       // Note that there is no interrupt unsafety here.  The IRQ can only ever change .enabled from 1->0
       // We're also doing that, so even if an IRQ occurred it would still stay as 0.
       waveform[i].enabled = 0;
-      int cnt = timer1CB?1:0;
-      for (size_t i = 0; i < countof(waveform); i++) {
+      int cnt = timer1CB ? 1 : 0;
+      for (size_t i = 0; (cnt == 0) && (i < countof(waveform)); i++) {
         cnt += waveform[i].enabled ? 1 : 0;
       }
       if (!cnt) {

cores/esp8266/core_esp8266_wiring_digital.c

@@ -89,7 +89,6 @@ extern void ICACHE_RAM_ATTR __digitalWrite(uint8_t pin, uint8_t val) {
 }
 
 extern int ICACHE_RAM_ATTR __digitalRead(uint8_t pin) {
-  stopWaveform(pin);
   if(pin < 16){
     return GPIP(pin);
   } else if(pin == 16){